London: If you suddenly find your partner has started going to the gym to shed the flab, don't feel too happy about it -- there may be a crisis in your relationship, say researchers.

A new study has found that people who suddenly start doing workouts to lose weight, while in an apparently steady relationship, are secretly preparing to dump their other partners, a daily reported.

In fact, happy couples don't worry about their expanding waistlines because there is less pressure to look attractive, say the German researchers.

Lead researcher Professor Thomas Klein studied the link between happiness and body weight and said the warning signs could include a sudden obsession with the gym or getting involved in sports and diets.

"When you are on the lookout for a new partner people try to be as thin and attractive as possible," Prof Klein of the University of Heidelberg was quoted as saying.

Consequently, when one half in a relationship starts slimming down it can be interpreted as "preparing for the partner market".

Prof Klein added: "People in a relationship feel less pressure and often put on weight as they do not watch their weight so much. In a happy partnership, people tend to get fat. With less competition, the weight difference is low."

But a crisis in a relationship can end the gluttony; and that could end up with both partners dropping weight, he says.

For their study, the researchers studied more than 2,000 people aged between 16 and 55. Among the findings were people in couples generally weighed more than single people hoping to attract a partner in a competitive dating market.

A separate US report found that married couples put on weight through "marital synchronicity" -- a shared environment of too much calorie-rich food and not enough exercise.

"Once the ink on the marriage contract dries up, they relax the control they had on their weight and appearance," said the researchers.